A nitride semiconductor light-emitting element prepared by employing an AlGaInN-based nitride semiconductor is capable of emitting short-wavelength light such as blue light in high efficiency, and hence a light-emitting device emitting white light can be obtained by combining the same with a fluorescent material. A light-emitting device overstriding luminous efficiency a fluorescent lamp is increasingly obtained as a light-emitting device emitting white light, and hence such a light-emitting device is considered as playing the leading part in future illumination.
As to a light-emitting device emitting white light with such a nitride semiconductor light-emitting element, on the other hand, further improvement of luminous efficiency and resulting development of energy saving are expected.
The principle of light emission in the nitride semiconductor light-emitting element resides in recombination of holes and electrons, and hence it is important to properly prepare a p-type nitride semiconductor layer and an n-type nitride semiconductor layer. In a production process for the nitride semiconductor light-emitting element, however, there are various heat treatment steps disturbing distribution of a p-type impurity such as Mg doped into the p-type nitride semiconductor layer.
First, there is an epitaxial growth step of forming the p-type nitride semiconductor layer itself, which is generally carried out at a high temperature exceeding 1000° C. Second, there is an annealing step for prompting control of the p-type nitride semiconductor layer to the p-type. Third, there is a heat treatment step for improving contactness between an electrode and a nitride semiconductor and the characteristics of the electrode itself after formation of the electrode.
As a result of the heat history in these heat treatment steps, difference arises between the doping quantity of the p-type impurity during epitaxial growth of the p-type nitride semiconductor layer and the doping profile of the p-type nitride semiconductor layer obtained by the doping of the p-type impurity.
Various technical developments for improving the characteristics of a nitride semiconductor light-emitting element by solving such a problem are conducted.
In PTL 1 (Japanese Patent Laying-Open No. 2009-130097), for example, there is disclosed a technique of forming a multilayer structure of an Mg-doped p-type Al0.15Ga0.85N blocking layer and an Mg-doped p-type GaN layer on an active region.
In PTL 2 (Japanese Patent Laying-Open No. 2000-164922), for example, there is disclosed a technique of improving contactness with a positive electrode by increasing an Mg impurity concentration in an outermost portion of a p-type contact layer in contact with the positive electrode.
In PTL 3 (Japanese Patent Laying-Open No. 2001-148507), further, there is disclosed a technique of bringing a p-type nitride semiconductor layer on an active layer into a structure obtained by stacking three layers including a p-type cladding layer doped with a p-type impurity in a middle concentration, a p-type low-concentration doping layer doped with the p-type impurity in a low concentration and a p-type contact layer doped with the p-type impurity in a high concentration in this order.
In Example 7 of PTL 3, there is disclosed a technique of forming a p-type cladding layer made of p-type Al0.16Ga0.85N doped with Mg by 5×1019 atoms/cm3 as a p-type layer doped with Mg in a middle concentration, forming a low-concentration doping layer made of undoped GaN as a p-type layer doped with Mg in a low concentration and forming a high-concentration doping layer doped with Mg by 1×1020 atoms/cm3 as a p-type layer doped with Mg in a high concentration.    PTL 1: Japanese Patent Laying-Open No. 2009-130097    PTL 2: Japanese Patent Laying-Open No. 2000-164922    PTL 3: Japanese Patent Laying-Open No. 2001-148507